This application claims priority under to prior application JP 2002-76318, the disclosure of which is incorporated herein by reference.
The present invention relates to an electronic device for supplying a DC power from a DC power source to a load circuit and, in particular, to such a DC power supplying electronic device having a noise filter mounted on a circuit board.
Recently, a digital circuit technology including an LSI (Large Scale Integrated circuit) has widely been used in various applications such as computers, communication apparatuses, household appliances, car-used apparatuses, and so on. In those applications, there is an electromagnetic interference (EMI) problem that the digital circuit used generates a high frequency noise current, which flows through a power line to a power source. The power line may radiate an undesired electromagnetic wave as a noise. And the noise current may also flow into other circuits connected to the power source.
In order to settle the EMI problem, it is effective to the, so called, power decoupling technique, where the noise current from the digital circuit such as an LSI chip is eliminated, attenuated or reduced. In other words, the power source and the power line are isolated from the LSI chip in the high frequency. In the power decoupling technique, a noise filter is used as an existing decoupling chip, which comprises a capacitor of a various type. This is shown in, for example, FIG. 5 of JP-A 2000-77852.
Referring to FIG. 1, a capacitor 25 as the noise filter is connected between a ground conductor layer (GND layer) 8 and a power line conductor layer (VCC layer) 7 in a circuit board 6, on which an LSI chip 20 is mounted and is connected to the VCC layer 7 and the GND layer 8. The VCC layer 7 and the GND layer 8 are connected to a DC power line (not shown) and the LSI chip 20 are therefore supplied with a DC power from the DC power source through the VCC layer 7 and the GND layer 8. In the circuit structure, the VCC and GND layers 7 and 8 are short-circuited in the high frequency by the capacitor 25. The high frequency noise current flows from LSI chip 20 through a loop shown by an arrow of a solid line and is eliminated from the power line circuit comprising the VCC layer 7 and GND layer 8. However, it is difficult for the capacitor 25 to retain a low impedance to an extent of a higher frequency. This is because of a self-resonance phenomenon of the capacitor. Therefore, when the frequency of the noise current becomes higher and higher, the impedance of the capacitor 25 becomes higher so that the noise current does not flow through the capacitor 25 but flows through the VCC layer 7 and the GND layer 8 extending over the entire region of the circuit board 6, as shown by another arrow of dotted line, and therefore to the power source (not shown). Thus, undesired electromagnetic wave is radiated, unfortunately. Therefore, the existing noise filter such as capacitor is not effective for the power decoupling to attenuate the noise current in a recent tendency where a signal transmission speed is increased more and more in the digital circuit.
Therefore, in use of the digital circuit operated at an increased high speed, a noise filter is required, which has a low impedance even at an elevated high frequency. Such a noise filter as required is proposed by two of the present joint inventors together with a different joint inventor in a prior Japanese Patent Application No. 2001-259453 filed on Aug. 29, 2001 (published under No. JP-A 2002-164760 on Jun. 7, 2002), which is assigned to the same assignee of the present application. The noise filter proposed in the prior patent application is a capacitor in the structure but is a noise filter of a distributed-constant circuit type at a high frequency. Therefore, the proposed noise filter can be used in the manner similar to the capacitor 25 in FIG. 1.
Referring to FIG. 2, it will be described as to an example using the proposed noise filter 1 of the distributed constant circuit type in place of the existing noise filter 25 comprising a capacitor on a circuit board similar to the circuit board 6 shown in FIG. 1. The noise filter 1 and a load circuit 20 are mounted on the circuit board 6. The noise filter 1 is provided with power input, ground input, power output, and ground output terminals 2, 3, 5, and 4.
The circuit board 6 is different from the circuit board of FIG. 1 and have first to fourth conductor lands 13, 14, 15, and 16 to which the power input terminal 2, ground input terminal 3, ground output terminal 4, and power output terminal 5 are connected, respectively. First and fourth conductor lands 13 and 16 are connected to VCC layer 7 through through-holes 9 and 12, respectively. Second and third conductor lands 14 and 15 are connected to the GND layer 8 through through-holes 10 and 11, respectively.
In the structure shown in FIG. 2 using the proposed noise filter 1, there is still a problem that it is insufficient in reduction of the high frequency noise, because of the reasons described below.
In the circuit board 6, VCC layer 7 and the GND layer 8 are usually formed with wide and continuous patterns, respectively. Therefore, the impedances of the VCC layer 7 and the GND layer 8 are generally very low. Contrastively, the impedances of the through-holes 9-12 connecting VCC layer 7 and GND layer 8 to the first to fourth conductor lands 13-16 are generally higher than those of the VCC and GND layers 7 and 8. Furthermore, the impedances of the through-holes 9-12 tend to become higher according to elevation of a frequency of the noise current.
Therefore, as the frequency of the noise current becomes higher and higher, the noise current is difficult to flow into the noise filter 1 through through-holes 9-12. Thus, the noise current is not reduced but transmitted through the VCC layer 7 and GND layer 8.
In view of the above-mentioned circumstances, an object of the present invention is therefore to provide an electronic device for supplying a DC power from a DC power source to a load circuit, which can sufficiently reduce a high frequency noise generated from the load circuit.
The present invention provides an electronic device for supplying a DC power to a load circuit from a power source and isolating in high frequency a power line circuit including the power source from the load circuit generating a high frequency noise. The electronic device comprises a circuit board and a noise filter. The circuit board comprises first through fourth conductor lands formed on a surface of the circuit board, a power line conductor layer connected to the first conductor land, and a ground conductor layer connected to the second conductor land. The power line conductor layer is adapted to be connected to the power source. The ground conductor layer is adapted to be grounded. The noise filter is mounted on the circuit board and has a power input terminal, a ground input terminal, a ground output terminal, and a power output terminal, which are connected to the first through the fourth conductor lands, respectively. The noise filter has a high frequency filtering circuit for reducing the high frequency noise incoming thereto but allowing a DC current to pass therethrough. The device further comprises power and ground conductors. The power and the ground conductors are connected to the fourth and the third conductor lands for connecting the third and the fourth conductor lands with the load circuit without connecting to the power line conductor layer and the ground conductor layer.
The ground input terminal and the ground output terminal may be joined to each other to form a single ground terminal.
The power line conductor layer may be formed, as an inner layer, in the circuit board and be connected to the first conductor land through a first through-hole formed in the circuit board.
The ground conductor layer may be formed, as an inner layer, in the circuit board and be connected to the second conductor land through a second through-hole formed in the circuit board.
The power line conductor layer may be formed on the surface of the circuit board and connected on the first conductor land.
The ground conductor layer may be formed on a rear surface of the circuit board and be connected to the second conductor land through a third through-hole formed in the circuit board.
The power conductor may be formed, as an inner layer, in the circuit board and be connected to the fourth conductor land through a fourth throughhole formed in the circuit board.
The ground conductor may be formed, as an inner layer, in the circuit board and be connected to the third conductor land through a fifth through-hole formed in the circuit board.
The power conductor layer may be formed on the surface of the circuit board and connected to the fourth conductor land.
The ground conductor may be formed on a rear surface of the circuit board and connected to the third conductor land through a sixth through-hole formed in the circuit board.
The load circuit may be mounted on the surface of the circuit board and is connected to the power conductor and the ground conductor.
The load circuit may be mounted on a different circuit board separate from the circuit board. The power and the ground conductors may include a power conductor pin and a ground conductor pin connected to the fourth and the third conductor lands, respectively, on the circuit board for connecting the fourth and the third conductor lands with the different circuit board to establish an electrical connection of the fourth and the third conductor lands to the load circuit.
The load circuit may be surface-mounted on the different circuit board. Furthermore, the power and the ground conductor pins may penetrate in the different circuit board from a rear surface to the surface thereof.
The noise filter may be a distributed constant type.
Still another objects, features, and advantages of the present invention will become apparent upon a reading of the following detailed description.